The vast majority of synaptic transmission in the mammalian nervous system is mediated by neurotransmitter messengers. Propagation of this information depends upon the transfer of the stimulus from the receptor to intracellular effectors. The hormonal stimulation of adenylate cyclase activity is a model system of receptor-effector coupling that has been studied extensively. Until recently, however, much less attention has been directed toward hormonal inhibition of adenylate cyclase and the related effects of guanyl nucleotides on the receptor binding of agonists. Recent studies in many laboratories have suggested that separate regulatory subunits mediate the stimulatory and inhibitory hormonal regulation of adenylate cyclase (Ns and Ni respectively) and that there may also be a separate subunit for the regulation of the binding of agonists. In a series of studies in intact human platelets, we have found the regulation of the binding of alpha2 agonists to be functionally dissociated from the inhibition of adenylate cyclase that is produced by these agonists. Additionally, maximal inhibition of adenylate cyclase occurs at low levels of occupancy of alpha2-adrenergic receptors. This finding is reminiscent of studies in stimulatory systems, and we have obtained direct evidence for the presence of spare receptors in our system. In the present grant period, I expect to further characterize the agonist-subpopulations of alpha2 adrenergic receptors of intact platelets by pharmacological and bichemical means to determine: (1) whether the subpopulations are readily interconverted in the absence of guanyl nucleotides; and (2) which subpopulation(s) is related to the inhibition of adenylate cyclase. The findings from these studies may have relevance to other receptor systems which are inversely coupled to adenyate cyclase. Additionally, a concurrent computer-simulation of spare receptor theory will provide theoretical guidelines to the study of the receptor reserve of intact platelets; in turn, the experimental study will provide empirical guidelines for the simulation. Recent clinical investigations in our laboratory, as well as others, have examined the alpha2 adrenergic receptor in the platelet as a biological marker in major depressive disorders. The studies described in this proposal will increase our understanding of alpha2 receptors in the platelet and will contribute to complex and novel approaches to the use of platelet alpha2 receptors as biological markers in clinical studies of neuropsychiatric disease.